1. Field of the Invention
The invention relates to a method which is intended for producing a pipe from a band which consists of a layer of a first metal and of a layer of a second metal, the layers being metallurgically bonded to one another over the entire length of the band, and by means of which the band, running in longitudinally, is formed into an open-seam pipe such that its longitudinal edges butt against one another along a longitudinally extending slit, and by means of which the slit is welded in order to form a pipe which is closed all the way round.
The term ‘metal’ will be understood hereinbelow likewise to mean the term “alloy”.
2. Description of Related Art
Metallic pipes made of two layers of different metals have various applications, in particular in the cable industry and for transporting gases and liquids. In the case of such composite pipes, the desired properties of the metals used can advantageously be utilized equally. For example it is possible for a pipe to be plated on the outside for corrosion-prevention reasons or to provide better heat conduction. Electrically conductive cables may consist of the inexpensive basic material aluminum with a layer of copper, which is a good electric conductor, plated thereon. Such CCA (copper-clad aluminum) pipes are particularly effective, by virtue of the skin effect, as high-frequency conductors.
Known methods of producing such metallic pipes are based on electroplating or mechanical bonding techniques.
A metallic layer can be electroplated on a metal pipe, as is done, for example, in the production of galvanized steel pipes. This gives rise to an unbreakable bond between the two metal layers. Electroplating is an energy-intensive process and is associated with relatively low production speeds.
The mechanical methods include, for example, plating. A flat metallic band has a thin flat band made of some other metal rolled onto it. A metal strip plated in this way can then be formed into a pipe and welded. One disadvantage of this method is that there is no self-contained, uniform outer layer present in the weld-seam region of the pipe, since the two metals mix during welding. As a result, it is not possible for the material properties of the pipe to be set in a controlled manner in the weld-seam region and these material properties can differ considerably from those of the rest of the pipe. Moreover, mixing may result in embrittlement of the weld seam.
EP 1 469 486 B1 describes a copper-coated aluminum strip which can be used for example for producing coaxial cables. The copper-plated band is produced, for a desired application, such that the aluminum strip is somewhat wider than the copper strip. During the operation of welding the band which is formed into a pipe, it is only the aluminum which is welded. However, this results in the copper layer having a gap along the aluminum weld seam. The conductive properties of the finished cable are therefore impaired particularly in the high-frequency range. Furthermore, the aluminum is exposed to corrosion in the weld-seam region.